package httpr

import (
	"context"
	"errors"
	"net/http"
	"strings"
)

type path []string

// newPath splits a path and ensures that it starts with a slash (/) and doesn't
// have more than 1 catch-all parameter.
func newPath(path string) (path, error) {
	if path[0] != '/' {
		return nil, errors.New("path should start with a slash (/) symbol")
	}

	if strings.Count(path, "*") > 1 {
		return nil, errors.New("there can be only one catch-all (*) parameter in path")
	}

	parts := strings.Split(strings.TrimSuffix(path, "/"), "/")

	parts[0] = "/"

	return parts, nil
}

// Params holds path parameters that are set as :key.
type Params map[string]string

type paramsKey struct{}

// ParamsKey is used as a key for Params in a request's Context.
var ParamsKey paramsKey = paramsKey{}

type node struct {
	endpoint string
	children []*node
	handler  http.HandlerFunc
}

func (n *node) get(path path, idx int) (http.HandlerFunc, Params) {
	// Check if this node is a catch-all endpoint.
	if n.endpoint[0] == '*' {
		var p Params = Params{}
		p[n.endpoint[1:]] = strings.Join(path[idx:], "/")
		return n.handler, p
	}

	// If this endpoint is a parameter, then add its name to a path's part.
	// This will be used further to fill Params.
	if n.endpoint[0] == ':' {
		path[idx] = n.endpoint + ":" + path[idx]
	}

	if len(path) == idx+1 {
		var params Params = make(Params)

		for _, part := range path {
			if part[0] == ':' {
				param := strings.Split(part[1:], ":")
				params[param[0]] = param[1]
			}
		}

		return n.handler, params
	}

	if len(path) > idx+1 {
		var wildcardOrParam *node
		for _, next := range n.children {
			if next.endpoint == path[idx+1] {
				return next.get(path, idx+1)
			}

			if next.endpoint[0] == ':' || next.endpoint[0] == '*' {
				wildcardOrParam = next
			}
		}

		if wildcardOrParam != nil {
			return wildcardOrParam.get(path, idx+1)
		}
	}

	return nil, nil
}

func (n *node) add(path path, idx int, handler http.HandlerFunc) error {
	// If it is a last part of path, then set a handler to this node.
	if len(path) == idx+1 {
		n.endpoint = path[idx]
		n.handler = handler
		return nil
	}

	// Check if next part is a parameter and if it is, then look for
	// an already existing endpoint with a different key.
	if path[idx+1][0] == '*' || path[idx+1][0] == ':' {
		for _, child := range n.children {
			if (child.endpoint[0] == '*' || child.endpoint[0] == ':') && path[idx+1] != child.endpoint {
				return errors.New("there is already a catch-all or regular param in there! You cannot add a second one")
			}
		}
	}

	// Check for an already existing endpoint.
	for _, child := range n.children {
		if child.endpoint == path[idx+1] {
			child.add(path, idx+1, handler)
			return nil
		}
	}

	// No endpoint was found.
	new_child := &node{endpoint: path[idx+1]}
	new_child.add(path, idx+1, handler)
	n.children = append(n.children, new_child)

	return nil
}

type Router struct {
	tree            map[string]*node
	NotFoundHandler http.HandlerFunc
}

func New() *Router {
	return &Router{tree: make(map[string]*node)}
}

func (rr *Router) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if tree, ok := rr.tree[r.Method]; ok {
		if r.URL.Path[0] != '/' {
			panic("first element of path should be a slash (/) symbol")
		}

		path, _ := newPath(r.URL.Path)

		if handler, params := tree.get(path, 0); handler != nil {
			if params != nil {
				r = r.WithContext(context.WithValue(r.Context(), ParamsKey, params))
			}
			handler(w, r)
		} else {
			if rr.NotFoundHandler != nil {
				rr.NotFoundHandler(w, r)
			} else {
				http.Error(w, "Not Found", http.StatusNotFound)
			}
		}
	} else {
		http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
	}
}

// Handler registers a handler for provided pattern for a given HTTP method.
func (rr *Router) Handler(method, pattern string, handler http.HandlerFunc) error {
	path, err := newPath(pattern)
	if err != nil {
		return err
	}

	if rr.tree[method] == nil {
		rr.tree[method] = &node{endpoint: "/"}
	}

	if err := rr.tree[method].add(path, handler); err != nil {
		return err
	}

	return nil
}

// ServeStatic serves a given file system.
//
// Path should end with /*filepath to work.
func (rr *Router) ServeStatic(path string, root http.FileSystem) {
	fileServer := http.FileServer(root)

	rr.Handler(http.MethodGet, path, func(w http.ResponseWriter, r *http.Request) {
		r.URL.Path = Param(r, "filepath")
		fileServer.ServeHTTP(w, r)
	})
}

// Param returns a parameter (that is set like `/a/b/:key/d`) inside a path
// with a key or empty string if no such parameter found.
func Param(r *http.Request, key string) string {
	if params := r.Context().Value(ParamsKey).(Params); params != nil {
		return params[key]
	}

	return ""
}